1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and especially relates to an image forming apparatus, which appropriately adjust the excess and deficiency of electric potential after exposure caused by uneven electrification and uneven sensitivity on the surface of photoreceptor.
2. Description of the Related Art
An electrophotographic image forming apparatus (such as a copier, printer, facsimile, and the like) writes an electrostatic latent image by having the surface of photoreceptor (generally, drum-shaped photoreceptor) uniformly electrified to a prescribed initial electric potential by an electrification apparatus (electrification means), and by exposing the aforesaid electrified photoreceptor's surface by exposure means (such as the means for scanning laser source and its beam, and LED array).
When an image formation is conducted, firstly, the value indicating the shading level of each pixel (hereinafter referred to as “pixel gradation”) is determined based upon the image data of a target image formation by a prescribed image processing means, such that the surface of photoreceptor electrified by the electrification apparatus is exposed by exposing means according to the exposure amount obtained by transforming (normally, linear transformation) the aforesaid pixel gradation determined by the image processing means based upon the prescribed transformation data into the exposure amount.
Incidentally, each photoreceptor has a peculiar electric potential distribution, even if the surface of photoreceptor is uniformly electrified under a fixed condition by the electrification apparatus, since each photoreceptor has individual differences caused by unevenness of such as the film thickness and material property in its surface member. Also, due to a distribution in the electrification amount of the electrification device itself, there occurs a distribution in the electrification amount of the photoreceptor occurring in a longitudinal direction of the electrification device. This is so-called “uneven electrification”.
Even if each region having the same initial electric potential is exposed with the same exposure amount, the electric potential does not necessarily decrease to the same value, causing unevenness. In other words, this is the situation where occurs a distribution (unevenness) in the ratio of differences in the electric potential decrease to the exposure amount (in other words, a slope of a graph showing relationship between the exposure amount and the electric potential after exposure), and this is so-called “uneven sensitivity”. Furthermore, there also occurs an electric potential distribution in the photoreceptor, due to uneven exposure as a phenomenon wherein the exposure amount by the exposing means becomes uneven in the width direction of the photoreceptor. For example, uneven exposure (the electric potential difference) is likely to occur between the center and around the both edges in the width direction of the photoreceptor, due to optical property of the exposing means.
Thus, in regard to these respective regions of the surface of photoreceptors which have individually different uneven electrification and sensitivity, if the pixel gradation is transformed into the exposure amount based upon the same (common) transformation information (can be called “transformation factor”), even if each region is exposed with the same exposure amount, the electric potential after exposure differs in each region, thereby causing uneven development (uneven density) since the density developed with toner (development density) has the excess and deficiency against the proper density.
In general, in case of the apparatus which conducts gradation representation by the area coverage modulation method representing the image shading with arrays of the pixel gradation of multiple pixels (so-called “digital machine”), although minute uneven sensitivity and uneven electrification less appear as uneven density compared to the apparatus which represents image shading with only shading per pixel (so-called “analog machine”), the digital machine conducting gradation representation by area coverage modulation method is also unable to completely avoid uneven density when there exists large uneven electrification.
Especially, although the color image forming apparatus superimposing four color toner images of CMYK (Cyan, Magenta, Yellow and Black) forms a color-mixed gray image by superimposing 3 color toner images of CMY, an uniform color-mixed gray image can not be formed (uneven density occurs) when uneven electrification appears on the surface of photoreceptor after exposure, losing the balance in CMY.
For example, according to Japanese Patent Publication Number 2003-154706, it has been considered that uneven density prominently appears when the electric potential after exposure has more than 5V of uneven electric potential. Such phenomenon appears more prominent in the so-called “tandem-style color image forming apparatus”. Also, since uneven electrification of the a-Si photoreceptor (the photoreceptor comprising photosensitive layer made of amorphous silicon) is generally larger than that of OPC photoreceptor, the uneven density in images becomes more prominent. Even if the quality standard (acceptance level) for the a-Si photoreceptor is decided to have 5V or less uneven electrification, the yield significantly degrades, and that is not realistic.
Correspondingly, Japanese Patent Publication Number 2003-154706 discloses the skill that provides the auxiliary exposure means to correct distribution of the initial electric potential in the process of exposure for writing electrostatic latent image.
Also, the skill to correct the exposure amount based upon the information of the sensitivity of photoreceptor, the skill to correct the uneven sensitivity per rotational position of the photoreceptor, the skill to correct the uneven sensitivity per exposure position of photoreceptor, and the skill to correct uneven sensitivity according to the data of sensitivity distribution of photoreceptor, are disclosed respectively in Japanese Patent Publication Number 1998-31332, 2000-162834, 2004-61860 and 2004-233694.